A behavior genetics research design is used to examine early precursors of adjustment and maladaptive behavior patterns in monozygotic and dizygotic twins. The twine are studied from late infancy through early childhood in a collaborative study based in Colorado. The purpose is to assess genetic change and continuity in temperament, emotional organization, and cognition. Children are observed in structured and naturalistic situations in the home and laboratory. The specific goals of the NIMH component of the study are (l) to describe early development of prosocial and anti-social orientations, (2) to identify affective components that underlie these behavior, and (3) to estimate genetic and environmental factors that contribute to early individual differences. Intra-class correlations comparing concordance of mz and dz twins, in conjunction with model-fitting estimates of heritability, suggest genetic influence at 14 and 20 months of age for measures of prosocial concern. Evidence is stronger for the earlier age period. Environmental factors appear to play a role in patterns of change in children's empathy over during this period of time. Early maternal warmth predicts maintenance of high levels of empathy from 14 to 20 months of age. Children who show decreases in empathic concern over time were from homes where the family environment was more negatively expressive and controlling, and less cohesive. Girls appear to be particularly affected by adverse family conditions.